1. Field of the Invention
The invention concerns a system of mirrors for guiding an electromagnetic (EM) wave. One application for such a system is in the field of plasma physics, where it can be used to guide a millimeter or submillimeter wave from a generator to a plasma machine for various purposes (ionization, heating, measurements of density or stability, etc . . .).
2. Discussion of Background
When the wavelength is greater than 3 mm, oversized waveguides are generally used. It is possible to maintain the wave propagation in a chosen propagation mode, TE01, for example, over a distance which can be as great as several tens of meters.
For shorter wavelengths, it has been demonstrated that systems of mirrors are more advantageously used, even though they are bulkier. This is related to the fact that the energy losses of an EM beam are independent of the diameter of the beam, whereas the distance between the mirrors increases with the surface area of the mirrors for a given maximum loss due to diffraction.
The known prior art is illustrated in FIG. 1. Such a system is described in the article by Mr. G. Faillon and Mr. G. Mourier titled "Developments in Microwave Tubes for RF Heating of Plasmas" published in Proceedings of the 2nd Joint Grenoble-Varenna International Symposium, which was held at Olmo, Italy from Sept. 3 to 12, 1980.
In such a system, mirrors E1, E2, . . . with an ellipsoidal form are used. Successive images are formed within the ellipsoids, aligned along their common axis.
Although such a system is satisfactory from certain points of view, it is difficult to construct because of the ellipsoidal form of the mirrors.
In the same field of applications, there is also a need to modify the general direction of propagation of an EM wave: for example an EM wave may come out from a generator in a vertical direction whereas the enclosure for the use of the EM wave may be found at a horizontal distance from the generator.
In such a case, as shown in FIG. 2, one can also use an elliptical mirror M, which deviates the beam and reconcentrates it in a different direction.
One can use several mirrors of this type, placed end to end.
This solution, although simpler than the preceding one, still presents certain inconveniences. If the beam has a circular cross section, the form of the mirror must be more and more elliptical as the incident angle alpha increases. Furthermore, the focal length is not the same in the plane of the reflection as in the perpendicular plane. A parallel beam will converge at a distance R/2 cos (alpha) in the plane of reflection and at a distance R/2 in the perpendicular plane, where R is the radius of curvature.
In order to concentrate a beam at a single point at a distance f, it would in fact be necessary to use a mirror with two curvatures, equal respectively to: EQU R.sub.i =2f/cos (alpha)
in the plane of reflection, and EQU R=2f
in the perpendicular plane.
The difficulties of realization for such mirrors with two different radii of curvature are considerable, and the situation is not much improved with respect to that of ellipsoidal mirrors.